WLAN (wireless local area networks) systems are indispensable forms of communication systems like other known and emerging wireless technologies. While all these technologies seek to cater the telecommunication needs of mobile users, they differ in scope, coverage bandwidth and quality of service. WLAN systems are governed by 802.11 standards and are very much sought after, and offer unique features that are different from the features of other wireless systems. As known, 802.11a is the wireless LAN standard for 5 GHz band transmissions based on OFDM (orthogonal frequency division multiplexing). It is noted that 802.11b systems are governed by the 802.11b standard for 2.4 GHz band and are based on DSSS/CCK (direct sequence spread band/complementary code keying. It is also noted that 802.11g is an enhancement over the 802.11b standard operating also in 2.4 GHz. 802.11g supports transmissions of both 802.11a and 802.11b frames in 2.4 GHz band giving a maximum data rate of 54 Mbps. In 802.11 transmissions, it is common to correlate the received signal with a delayed copy of itself for frame detection. However, in 802.11g systems which can support both 802.11a and 802.11b frames, delayed copy correlation (DCC) alone will result in false frame detection, particularly in multipath channel conditions. The correlation properties of 802.11b frames which use Barker sequences for the preamble are more robust than the correlation properties of 802.11a preamble sequence. Barker codes are a subset of PN sequences and have low correlation side lobes. A correlation side-lobe is the correlation of a code word with a time-shifted version of itself. It is to be noted that the timeline available for frame acquisition in 802.11a packets (8 microseconds) is much smaller than that available in 802.11b packets (56/128 microseconds).
802.11a and 802.11b frame detection schemes have been discussed independently in literature but rarely under the assumption of simultaneous co-existence of the two kinds of frames, which is the case in 802.11g systems. Schemes for distinguishing between 802.11a and 802.11b packets based on 0.8 microseconds and 1 microsecond based DCC can be deployed but such schemes would not be very robust, particularly in multipath channel conditions. In this context, reference may be had to the publication of Nov. 18, 2003 titled “Improving Efficiency When Detecting WLAN Preambles,” authored by Richard Williams, and published in Communications Design. Other publications which may be referred to include: Timothy M. Schmidl and Donald C. Cox, Robust Frequency and Timing Synchronization for OFDM, IEEE Transactions on Communications, Vol. 45, No. 12, December 1997; and J. Heiskala and J. Terry, OFDM Wireless LANs: A Theoretical and Practical Guide, SAMS Publishing, 2002.